Hi (again) Peter,
One thing to bear in mind is that the North Pennines suffered massive
glacial erosion over the past 1.8 million years or so - South Devon did not.
I see this here in Wales - the "high-grade" supergene deposits -
Llechweddhelyg, Bwlchglas etc etc are a strict minority - flukes of not
being eroded away, for various reasons, while in the SW the oxidation zones
are still intact. They all formed - UK-wide - in the Tertiary, but ice
erosion destroyed all but the deepest in more recent times.
See: Mason, J.S. 2004. The development and preservation of supergene lead
mineralisation in Central Wales. UK Journal of Mines and Minerals, 24, 35-46.
When you have deposits remaining like these, yet in the Rheidol Valley, you
have fresh marcasite at outcrop, the evidence is pretty compelling! Same
goes for the Leadhills & Caldbecks....
Devon and Cornwall cannot be compared to the remainder of the UK
metalliferous mining districts on this basis alone, unless you are talking
about ancient oxidation/reduction zones such as the native copper/cuprite at
New Cliffe Hill Quarry in Charnwood.
Cheers - John
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